Read this before you fly again!

Fears about the risk of contracting potentially fatal blood clots - also known as economy class syndrome or DVT - were first sparked by the death of bride-to-be Emma Christoffersen.

The healthy 28-year-old from Wales collapsed in the arrivals hall at Heathrow after a 20 hour flight in October last year.

She had complained of feeling unwell on the last part of the 12,000-mile Qantas flight from Australia via Singapore. She had

developed a deep vein thrombosis and died before reaching hospital.

New research now shows that one in ten travellers on long-haul flights develops tiny blood clots which could put them at risk from life-threatening disorder.

The British research team have made the first scientific attempt to guantify the size of the problem.

So what causes deep vein thrombosis and if it has been around for so long why didn't we know about it before?

We've discovered that DVT is responsible for hundreds of deaths and has been known about for decades.

Around 10 passengers a year die from a blood clot in the lungs within minutes of arriving at Heathrow and it has been estimated by the Aviation Health Institute (AHI) that 30,000 Britons may be affected by flight-induced blood clots every year.

But many will not link the condition to air travel as it can take up to a fortnight to work its way up to the heart or lungs. Among victims, around half develop swelling in their legs, but for others there are no clear signs until a major blockage occurs.

Doctors believe that DVT is caused on long flights when the leg remains still or the seat in front restricts circulation. The lack of movement can result in pulmonary embolism - a clot, which can fatally block vital blood vessels to the lung.

After years of denials, a summit held earlier in the year and attended by 16 major airlines, concluded there was a 'probable link' between DVT and air travel.

This has led to a rash of advice and products to help flyers ensure that they don't suffer from DVT. Airlines also face a barrage of compensation claims.

An Australian law firm announced it was bringing, for the first time, a 'class suit' representing 800 people against several of the world's major airlines.

And travel insurer Club Direct has included a provision, which pays legal costs of up to £25,000 for passengers or their relatives to take legal action against airlines in the event of death or injury due to DVT.

British Airways were the first carrier to provide travel health information leaflets following the scare.

Leaflets carrying health tips including dealing with the risks of fatal blood clots are being issued with long-haul tickets.

But it has taken a long time for the industry to tackle DVT which was first recognised by a physician who travelled from Boston to Venezuela in 1946 on a non-stop flight lasting 14 hours.

The term economy class syndrome was coined by two British researchers, Symington and Stack in 1977.

They discovered that there was a greater likelihood for the condition to occur in flight than on ground based transport because of several contributing factors:

• immobility

• cramped seating

• compression of the thighs by seat edge

• dehydration caused by excessive alcohol consumption

• low cabin humidity - two per cent verses 50 per cent on the ground.

• lower pressure increases coaguability factors in the blood and can also occur in business class where the leg rest puts pressure on the calves.

In March experts from the World Health Organisation, WHO, reviewed the existing state of evidence on air travel and venous thrombosis and concluded that further research is needed.

A study is being set up to discover answers to:

Is there a link and if so what is the absolute risk?

What is the size of the problem?

Do factors such as cabin air pressure, cabin oxygenation and behavioural risk factors such as excessive use of alcohol or lack of exercise contribute.

Also last month, UK MP's Dr Ian Gibson, and John Smith, called for a public inquiry into air related DVT. A committee has been elected to represent the victims and their families in talks with the Government and the airlines.

Called VARDA (Victims of Air Related DVT Association) the organisation is chaired by Ruth Christoffersen - Emma's mother. They want to know how long the Government has been aware of it and why nothing has been done?

Who is most at risk?

Those at high-risk include people over 40, those who are overweight, with heart or lung disease or passengers who have recently had surgery. Among younger passengers, professional athletes, stomach ulcer victims and people with a family history of blood clots are in danger.

The Aviation Health Institute, is a charity that promotes health and the well being of airline passengers worldwide. It can be contacted via e-mail at feedback@aviation-health.org